Set an upload quota on single-site WordPress

Description

I was searching for a way to turn on upload space quota for single-site WordPress (not multisite) install. There does not seem to be one. I can't even emulate one with hooks, because in media.php 'upload_ui_over_quota' hook is multisite-specific and 'pre-upload-ui' does not allow me to emulate the same behavior.

I understand that need for upload space quota for single-site is not a very common scenario, but the relevant code is already there, only an alternative method of enabling it (e.g. a constant in wp-config.php) is needed.

attachment:24051.diff​ moves get_upload_space_available() and is_upload_space_available() out of ms-functions.php and into media.php. Wrap all existing checks in get_upload_space_available() in an is_multisite() conditional, and create a new filter for the return of get_upload_space_available(): upload_space_available.